It's our society's own fault that people put off having kids till "later". Biologically, we are quite capable of reproducing in our teenage years, but society tells us that we aren't ready then. We're not (psychologically) mature enough, not financially secure enough, not socially stratified enough to bring a new life into the world, and we believe it. In fact, so many of us believe it so strongly that we still believe it well past our 40s when the peak our our hill looms just an Ollie away. We're not ready, we haven't got the money, we still have dreams we want to pursue that will presumably get us out of this social rut we're in before Baby ("Wah! Wah!", not "ooh, baby...!") ties us down irrevocably, once and for all.
It's society's rules that we have to get an education, get a job, get a mortgage, get insurance, pay taxes and do our part as a consumer of resources before we're considered responsible enough to bring forth offspring. If we strictly follow these rules, we'll never get to spawn at all as we'll never have enough personal security to know when we have met the prerequisites.
No wonder teens act up the way they do. They are physically mature but society keeps them immature with a brick wall comprising rules, norms and expectations that stands in their path. The collision is seldom pretty. Teens wonder why their parents/teachers/authority figures are so stupid; their parents et al, wonder what past sins they are being punished for. What a mess.
Which is why Juno is such a fortunate pregnant teen. Her friends and family take her shocking news with stoic resignation, then put their full support behind her decision to carry her baby to term. Instead of seeing the situation as the end of the world, they see more excitement in the potential for new life.
Despite the mistake she has made with her social timing, Juno is never alone in her predicament, nor made to feel guilty about keeping her baby even though she is still attending school. While she still gets stares from strangers and schoolmates who make fun of her, the people that matter to her continue to shower her with love and encouragement, making her burden just that little easier to bear.
Teenage pregnancy is no laughing matter, no matter the light treatment the movie gives it. Prevention is still better than cure, but if regardless of our best efforts as caregivers and it happens, it would be wonderful if the family just deals with the situation instead of making it worse through blame and recrimination.
Ultimately, it's Juno's courage, her commitment to do the right thing, her strength of character and her intelligence that salvages a bad situation and make it work for the greater good. Juno's a lovely character and a totally watchable movie if just a touch idealistic.